In recent years, high definition televisions ("high vision"), which provide more impression and deeper presence feeling and higher audio quality than conventional televisions, have been developed. The MUSE system proposed by NIHON HOSO KYOKAI (NHK) is an example of the associated transmission system. In the MUSE system, an audio signal is time-division multiplexed during a video V-blanking period.
Now referring to the drawings, an explanation will be given of a circuit for demodulating an audio signal in the MUSE system.
FIG. 5 is a block diagram of a conventional audio signal demodulating circuit, FIG. 6 is a timing chart of an audio signal in the circuit, and FIG. 7 is a MUSE transmission signal format
In FIG. 5, numeral 2 is an input changing circuit for changing the gain of an input data signal depending on whether the input data signal is in an AM mode or a FM mode.
Numeral 3 is an audio low pass filter (LPF) circuit for producing an audio data signal upon receipt of an output signal from the input changing circuit 2. Numeral 4 is a ternary-to-binary converting circuit for converting an audio data signal output from the LPF circuit, which is a ternary two symbol audio data signal, into a binary three symbol audio data signal. Numeral 5 is a time-base extension circuit for receiving an audio data signal multiplexed during vertical blanking periods of a video signal and extending the time base of the received audio data signal to restore it to a continuous audio signal. Numeral 6 is an inter-frame de-interleave circuit for canceling the inter-frame de-interleave applied to an audio data signal at the transmission side. Numeral 7 is a timing signal generating circuit for producing several kinds of timing pulses on the basis of a frame pulse (FP) and a horizontal pulse (HP) supplied from an image synchronization circuit (not shown). Numerals 8, 9 and 10 are frequency division circuits for frequency-dividing an input clock.
The operation of the audio signal demodulating circuit as constructed above will be explained below.
First, an n-bit input audio data signal supplied to a data input terminal is changed in its gain corresponding to its mode by the input changing circuit 2, since the multiplexing level of an audio data signal differs between the AM mode and the FM mode. The data signal, which has thus been changed to have an audio multiplexing level common to the AM mode and the FM mode, is supplied to the LPF circuit 3 The LPF circuit 3 constitutes a digital LPF circuit having 6 MHz root cosine roll-off characteristic which serves as an audio low pass filter. The ternary-to-binary converting circuit 4 converts the audio data signal from a ternary two symbol signal into a binary three symbol signal. In this case, an audio signal has been time-multiplexed during the vertical blanking periods of a video signal, as illustrated in the MUSE transmission signal format shown in FIG. 7, to have a time-base arrangement shown at (E) of FIG. 6. The time-base extension circuit 5 extracts this time-multiplexed audio signal and extends its time base to provide a continuous audio signal as shown at (D) of FIG. 6.
Then, since the input audio signal has been previously inter-frame interleaved at the transmission side in order to disperse a possible burst error occurring in the transmission system, the inter-frame de-interleave circuit 6 operates to cancel the inter-frame interleave at the receiving side. Thus, a continuous audio signal is outputted from a data output terminal as an output data.
Additionally, the timing signal generating circuit 7 generates and supplies several kinds of timing signals synchronized with frames of a video signal, such as used for extracting an audio signal in the time-base extension circuit 5, on the basis of a frame pulse (FP) and a horizontal pulse (HP) supplied from the video synchronization circuit (not shown). Further, the frequency division circuits 8, 9 and 10 frequency-divide a basic clock (=16.2 MHz) supplied to a clock input terminal from the outside and produce several kinds of clocks having the frequencies of 2.025 MHz, 1.35 MHz, etc.
However, a conventional audio signal demodulating circuit of the above construction comprises many components operating at a higher clock frequency (16.2 MHz), so that it consumes undesired excessive electric power.